Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g. from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development.
For variable displacement engines (VDEs), or other engines configured to run with one or more cylinders deactivated, the engine may generate less heat during a vehicle run-time than for an engine that operates with all cylinders constantly active. However, the entry conditions and thresholds for a typical EONV test are based on an inferred amount of heat rejected into the fuel tank. The inferred amount of heat may be based on engine run-time, integrated mass air flow, etc. For VDEs, these indicators may thus overestimate the amount of heat generated by the engine. As such, an EONV test may be initiated even if the engine has minimal cylinder activation time, leading to aborted or indeterminate test results. Further, with a reduced amount of rejected heat, the fuel tank may fail to reach EONV test thresholds, leading to false failures even if the fuel system is intact.
The inventors herein have recognized the above issues and have developed systems and methods to at least partially address them. In one example, a method, comprising: adjusting an evaporative emissions leak test parameter based on a ratio of cylinder run time of a deactivatable cylinder of an engine, to vehicle run time; and indicating degradation based on the adjusted parameter. The adjusted parameter may thus more accurately reflect the state of a vehicle configured to run with one or more cylinders deactivated. In this way, the evaporative emissions leak test may realize improved robustness and performance metrics, thus reducing warranty costs associated with poor test metrics.
In another example, a vehicle system, comprising: an engine comprising one or more selectively operable cylinders; a fuel system isolatable from atmosphere via one or more valves; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: adjust one or more thresholds for an engine-off natural vacuum test based on a ratio of cylinder run time to total vehicle run time; following a vehicle-off event, isolate the fuel system from atmosphere; and indicate degradation of the fuel system based on the one or more adjusted thresholds. Engines configured to run with one or more cylinders deactivated may generate less heat over the course of operation than do engines configured to run with all cylinders activated constantly. By adjusting thresholds for an engine-off natural vacuum test, the expected changes in fuel tank temperature and pressure may more accurately reflect the state of the engine and an expected amount of heat rejected to the fuel tank. In this way, false failures may be reduced by adjusting the expected resulting fuel tank pressure.
In yet another example, a method for a vehicle fuel system, comprising: adjusting a pressure rise threshold and a vacuum threshold for an engine-off natural vacuum test based on a ratio of cylinder run time to total vehicle run time; following a vehicle-off event, closing a canister vent valve responsive to the ratio of cylinder run time to total vehicle run time being greater than an initiation threshold; monitoring a fuel tank pressure for a first testing duration; responsive to a fuel tank pressure reaching the adjusted pressure rise threshold during the first testing duration, indicating that the vehicle fuel system is intact; responsive to a fuel tank pressure failing to reach the adjusted pressure rise threshold during the first testing duration, coupling the vehicle fuel system to atmosphere; responsive to the fuel tank pressure decreasing to atmospheric pressure, isolating the vehicle fuel system from atmosphere; monitor a fuel tank vacuum for a second testing duration; responsive to a fuel tank vacuum reaching the adjusted vacuum threshold during the second testing duration, indicate that the vehicle fuel system is intact; and indicating degradation of the vehicle fuel system responsive to the fuel tank vacuum failing to reach the adjusted vacuum threshold during the second testing duration. By initiating the EONV test only when the ratio of cylinder run time to total vehicle run time is greater than an initiation threshold, the execution rate of the test may be increased. In this way, the test may only be initiated when a threshold amount of rejected heat energy is inferred.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.